I probably should have been more clear in the question - the part about changing the default action was a reference of where in XP you look to find what I'm looking for in 7.

What I'm trying to do is leave the default action the same (open) but add arguments to the command line (in particular i want VLC player to automatically open files with a 3000ms buffer which I can do in a per file basis manually with VLC with the command lin, but I want it to happen every time I double click a file)

You can do that with that reg hack. You just have to figure out the correct command string to do it.

So if you are trying to change the startup for a .avi.
doing assoc .avi will return the value .avi=VLC.avi

Go back to that same classes registry area and look for VLC.avithen from there go to shell>Play>Command
the default action for that should say:"C:\Program Files\VideoLAN\VLC\vlc.exe" --started-from-file "%1"

add in your switches between the %1 and the "
So (and I don't know the commands) but if the switch to set a 300ms pause was /300ms
the command would look like:"C:\Program Files\VideoLAN\VLC\vlc.exe" --started-from-file "%1 /300ms"

Again you may have to play around with it a little to get it working right, but that's the basics. My syntax may be slightly off but just play around and you'll get it. You can't muck anything up. If you are worried save that registry key first before you make any changes.

Right click on the file in question. Choose open with, select the program you want to be the default for that file time - note this will change it for all files of that extension. Then below the program selection box put a check mark in the 'always open with' box.